Cavitary histoplasmosis has been experimentally produced in rhesus monkeys infected intratracheally. Previous observations have suggested that delayed hypersensitivity (cellular immunity) to the organism is responsible for the pulmonary cavitation process and that the presence of circulating antibodies against the organism can block the development of cavities. The present proposal was designed to test this hypothesis and to determine experimentally the effects of immunologic manipulation on the development of pulmonary cavities.